there are lots but many can become a nuisance over time. I've used Lysimachia mummularia,[creeping jenny] in a yellow bed. A range of Lamiums [dead nettles many variegated leaves with pink or white flowers or a rampant yellow one.] Vinca minor [periwinkle]. If shady then mitella is a good plant. Dianthus deltoids are a good carpeter.
I also use annuals for this purpose too, lobelia, alyssium, Limnanthes, Mesembryanthemums, annual phlox, nemmesia, diascia etc.

Hope this gives you some ideas. I'm sure others will suggest other things too.

Ajuga is another one, comes indifferent leaf colours and nice flower spikes
Do you have the low growing hardy geraniums? I find them easy to look after
Hardy annuals sown direct in the spring on bare patches is another option.

If you have a neutral to acid soil then how about Cornus Canadensis. A creeping herbaceous perennial forming a wide mat of oval leaves which turn red in autumn, about 4 to 6 inches in height. White flowers (bracts) in late spring and early summer, followed by red berries. Good in sun or shade.